The next commit is going to perform clipping when transforming
Bezier strokes.  When parts of the stroke get clipped, the result
consists of multiple strokes.

Adapt gimp_stroke_transform() in preparation, to allow for the
transformation to result in multiple strokes, by adding a GQueue*
parameter that receives the transformed strokes.

For convenience, we allow passing NULL as the argument, in which
case the current behavior is maintained.  However, NULL should only
be passed when clipping is known to be unnecessary.

Adapt the rest of the code for the change.

Note that this technically affects public API: existing stroke
object IDs now become invalid after transforming their containing
vectors object.  However, this is unlikely to affect code in
practice.